The SPM is an apparatus which scans a probe located in proximity to a surface of a sample, detects the action (interatomic force, tunnel current, or the like) acting between the probe and the sample, and can observe a shape of the surface of the sample by imaging the action. FIG. 4 is a diagram illustrating an example of an overall schematic configuration of the AFM which is one type of the SPM. An AFM (SPM) 1 includes a stage 11, a cantilever 12, a probe 13, a laser beam source (laser diode) 14, a collimator lens 15, a focus lens 16, a beam splitter 17, a mirror 18, a detector 19, and an optical microscope 20 (for example, refer to Patent Literatures 1 and 2).
A sample S which is an object of a surface observation is placed on the stage 11. The stage 11 can be driven in a height direction (Z direction) and a sample surface direction (XY direction) by a tube scanner (not illustrated) which is a driving mechanism, thereby moving the sample S in a three-dimensional (XYZ) direction.
The probe 13 is attached to a free end side of the cantilever 12. When the probe 13 approaches the surface of the sample S, interatomic force (attractive force or repulsive force) acts between the sample S and the probe 13. A force which tends to deflect the cantilever 12 is applied by the interatomic force.
On the other hand, a condensing optical system 21 including the laser beam source 14, the collimator lens 15, and the focus lens 16 is integrally held inside a cylindrical lens barrel 22, and the laser beam emitted from the laser beam source 14 is collected by the collimator lens 15 and converted into a parallel beam. Thereafter, the laser beam is condensed by the focus lens 16. The laser beam condensed in this manner is irradiated on a reflecting surface of the cantilever 12 and reflected. Further, the laser beam is reflected by the mirror 18 and guided to the detector 19.
For example, a quadrant photodiode is used as the detector 19. When the position of the reflected beam incident on the detector 19 changes due to deflection of the cantilever 12, the detector 19 can detect the variation and output the detected variation as a feedback signal. Further, by performing the feedback control on the drive mechanism (a Z-axis adjustment mechanism of a tube scanner) using the feedback signal so that a deflection amount of the cantilever 12 is kept constant, that is, the distance between the probe 13 and the surface of the sample S is kept constant, the intensity of the feedback signal becomes a change amount which reflects the unevenness of the sample S. Therefore, by forming an image on the basis of the feedback signal, the unevenness of the surface of the sample S is imaged.
Incidentally, after the laser beam passes through the focus lens 16, an optical axis direction of the laser beam is bent by the beam splitter 17 so that an optical axis O of the optical microscope 20 and an optical axis L of the laser beam overlap with each other.
The optical microscope 20 is used when a user of the AFM (SPM) 1 observes the sample S. However, in addition to this, the optical microscope 20 is also used when adjusting the irradiation position of the laser beam from the condensing optical system 21 to the cantilever 12.
In other words, the condensing optical system 21 is assembled so that the respective elements of the laser beam source 14, the collimator lens 15, and the focus lens 16 are integrated in the cylindrical lens barrel 22 in a state in which the optical axis is adjusted. The adjustment operation inside the lens barrel 22 is performed by a technician of the apparatus manufacturer in advance and is fixed so that a positional relation between the elements does not change after adjusted once. The lens barrel 22 is supported by a microscope head (not illustrated) serving as a support, and is attached to a position adjustment mechanism (not illustrated) which moves the entire lens barrel 22 in a two-dimensional direction (or a three-dimensional direction also including the optical axis L direction) orthogonal to the optical axis L.
By moving the lens barrel 22 (the condensing optical system 21) in the two-dimensional (or three-dimensional) direction using a laser beam adjusting knob (not illustrated) of the position adjustment mechanism, while looking into the optical microscope 20, the user of the AFM 1 can perform the adjustment which matches the irradiation position of the laser beam with the reflecting surface of the cantilever 12. Incidentally, the position adjustment of the lens barrel 22 by the user is performed by moving the entire lens barrel 22, and adjustment of the optical axis between the elements inside the lens barrel 22 on the user side is not performed as described above.
Here, the internal structure of the condensing optical system 21 of the AFM (SPM) 1 and the adjustment operation inside the condensing optical system 21 performed by the technician of the apparatus manufacturer will be described. In the ideal condensing optical system 21 of the AFM 1, since the condensing properties of the laser beam affect the performance of the AFM 1, when the user performs the adjustment so that the laser beam is irradiated on the reflecting surface of the cantilever 12 by the aforementioned laser beam adjusting knob, it is desirable that an irradiated spot area be as small as possible.
In order to construct the aforementioned ideal condensing optical system 21, adjustment is performed by a technician of the apparatus manufacturer so that a beam emitting point 14a (see FIG. 5) of the laser beam source 14 coincides with the center of the collimator lens 15 on the optical axis L, and it is necessary that the distance between the laser beam source 14 and the collimator lens 15 can be adjusted so that ideal collimating beam (parallel beam) is formed in the lens barrel 22.
For that purpose, if a plurality of fine adjustment mechanisms is provided inside the condensing optical system 21 so that the distance between the elements and the angle of the elements can be freely adjusted, the ideal condensing optical system 21 can be reliably constructed.
However, in recent years, it has been required to make the size of the entire SPM (AFM) apparatus compact as much as possible. For example, also regarding the condensing optical system 21 which condenses the laser beam on the reflecting surface of the cantilever 12, since an attachment space thereof is specifically limited to about 3 to 5 cm, it is practically difficult to incorporate a plurality of fine adjustment mechanisms. Furthermore, since it is required to reduce not only the size of the apparatus but also the cost, it is desirable to provide a simplified configuration without immoderately providing an adjustment mechanism.
For this reason, in order to minimize the adjustment mechanism in the condensing optical system 21, a simple adjustment mechanism for adjusting only the position of the collimator lens 15 on the optical axis is adopted at present.
Here, FIG. 5 is a cross-sectional view illustrating a condensing optical system 21 equipped with a conventional simple adjustment mechanism. The condensing optical system 21 includes the lens barrel 22 which holds the laser beam source 14, the collimator lens 15, and a lens mount 23 which holds the focus lens 16.
The lens barrel 22 has a hollow cylindrical shape, and is designed on the premise that each optical component is arranged so that the axis of the central axis of the lens barrel 22 becomes the optical axis L of the condensing optical system 21.
That is, the laser beam source 14 is attached to one end of the lens barrel 22, and the beam emitting point 14a of the laser beam source 14 is fixed so as to be located on the optical axis L. Incidentally, a female screw 24 is engraved on the inner circumferential surface of the lens barrel 22.
The lens mount 23 forms a hollow cylindrical body, and a male screw 25 engraved on the outer circumferential surface thereof is screwed with the female screw 24 of the lens barrel 22, and the axis of the center axis of the lens mount 23 and the axis of the lens barrel 22 are designed to overlap with each other in a state in which the male screw 25 and the female screw 24 are screwed together.
Therefore, the axis of the central axis of the lens mount 23 is also designed to coincide with the optical axis L, and the collimator lens 15 is fixed to one end (the end portion on the side close to the laser beam source 14) in the tube of the lens mount 23, and the focus lens 16 is fixed to the other end thereof with an adhesive. The centers of the collimator lens 15 and the focus lens 16 are arranged on the optical axis L in a state in which the lens mount 23 is screwed to the lens barrel 22 in this manner.
Further, the screwing position of the male screw 25 with respect to the female screw 24 is adjusted by a technician of the apparatus manufacturer so that the position of the beam emitting point 14a of the laser beam source 14 in the direction of the optical axis L becomes a focal point of the collimator lens 15. As a result, the collimating beam (parallel beam) can be emitted by the collimator lens 15. After completion of the position adjustment of the collimator lens 15, the position of the lens mount 23 is fixed with respect to the lens barrel 22 by a set screw 26 passing through the side wall of the lens barrel 22 from the outer side of the lens barrel 22.
After completion of adjustment of the lens mount 23, laser beam (collimating beam) enters the focus lens 16, and the laser beam that has passed through the focus lens 16 is condensed at the focal position of the focus lens 16 on the optical axis L. Further, the focal position is set near the reflecting surface of the cantilever 12.
As a result, the user moves the lens barrel 22 using the laser beam adjusting knob so that the reflecting surface of the cantilever 12 comes to the focal position (or on the optical axis L in the vicinity of the focal point), thereby making it possible to perform measurement with a high S/N ratio.
[Patent Literature 1] JP-A-2012-225722
[Patent Literature 2] JP-A-2014-211372